Electrowinning from aqueous solutions using insoluble anodes is a well-established process for recovery of metals such as zinc, copper, nickel, cobalt, cadmium, manganese and others. The metal is electrodeposited at the cathode from a solution of one of its salts, most commonly a sulfate. Water is decomposed at the anode, which is usually made of lead or a lead alloy, oxygen is evolved and acid (hydrogen ions) is formed. The electrowinning reactions may be described generally by the following (wherein M represents any of the metals mentioned above): ##STR1##
For a sulfate solution, the overall reaction can be written: EQU MSO.sub.4 +H.sub.2 O.fwdarw.1/2O.sub.2 +H.sub.2 SO.sub.4 +M
The oxidation reaction at the anode is a "waste" reaction because no useful byproduct is produced.
The minimum electrical energy consumption for the electrolytic process is proportional to the reversible electromotive force (emf). The actual energy used corresponds to the operating cell voltage which is the sum of the reversible emf plus irreversible potential differences, namely the ohmic drops, and the anodic and cathodic overpotentials. The actual energy use is inversely proportional to the electrochemical current efficiency. In a typical modern plant, the average current efficiency is 90% and the energy consumption is 1.4 kWh/lb for Zn.
A large amount of the waste of this electrolysis energy is associated with the oxygen evolution reaction involved in water oxidation which takes place at the anode. Accordingly, the substitution of an anodic electrochemical reaction for this wasteful water oxidation reaction would result in significant energy savings. Thus a useful oxidation reaction which can be operated in the same electrochemical cell as one where metal electrowinning is occurring would be of tremendous benefit.
Prior art anodic substitution processes of water oxidation either involve tangential reactions associated with the processes which offset the energy savings claimed, involve processes which are not sufficiently understood to know where the savings may be, or involve the electrowinning from electrolytes other than sulfate which in turn have energy consumption problems associated with them. In addition, the prior art has given attention to lower voltage reactions rather than looking into increased efficiency by using side-by-side reactions, neither of which individually is lower in voltage than commercial half-cell reactions, but which together result in increased voltage drop and high energy savings.
For example, U.S. Pat. No. 4,431,496 to Remick discloses a process for electrolytic recovery of zinc wherein metallic zinc is deposited at the cathode while the anode is depolarized through oxidation of iodide ions to iodine, avoiding oxygen evolution at the anode. The iodide ions are chemically regenerated by extracellular oxidation of sulfur dioxide with water to produce iodide ions and hydrogen ions for recycle to the anode compartment: EQU I.sub.2 +2H.sub.2 O+SO.sub.2 .fwdarw.2I.sup.- +H.sub.2 SO.sub.4 +2H.sup.+
The overall electrochemical plus chemical reaction is: EQU ZnSO.sub.4 +2H.sub.2 O+SO.sub.2 .fwdarw.Zn+2H.sub.2 SO.sub.4
Although the anodic reaction for I.sub.2 oxidation to I.sub.2 is thermodynamically 0.7 volts less than the corresponding oxygen evolution reaction, much of the energy savings is offset in the energy cost for separating and concentrating the excess sulfuric acid produced in the anolyte. This occurs since twice as much acid is produced as metal in the overall reaction and half the acid must be removed to maintain a constant acid level. Thus, although there is a useful oxidation reaction, energy savings are offset in recovering the products.
U.S. Pat. No. 4,204,922 to Fraser et al teaches a process for simultaneous electrodissolution and electrowinning of metals from a cell comprising an anode and a cathode separated by one or more ion permeable membranes, the membrane being impermeable to the particulate solids in the suspension separating the anode and cathode compartments. The recovery of metals is accomplished from sulphide minerals rather than sulfate electrolytes. The problems associated with elimination of sulphide ore waste products result in much greater inefficiencies. In addition, since there is no hydrogen evolved in this reaction, no current can be carried with these ions resulting in a waste of energy efficiency.
Additional studies report the electrowinning of metals from chloride rather than sulfate solutions. These processes, however, also require the use of a depolarizing anode. In addition, metal which has been deposited from chloride solutions has been found to be needle-like and non-consolidated and to require modified handling procedures.
U.S. Pat. No. 4,268,363 to Coughlin discloses the electrochemical gasification of carbonaceous materials by anodic oxidation which produces oxides of carbon at the anode and hydrogen or metallic elements at the cathode. U.S. Pat. No. 4,405,420 to Vaughan teaches the same reaction catalyzed by an iron catalyst. In both patents the substitution of the reaction EQU C.sub.(s) +2H.sub.2 O.fwdarw.CO.sub.2 (g)+4H.sup.+ +4e.sup.- E.sup.o =0.21 volt
for the reaction EQU 2H.sub.2 O.fwdarw.O.sub.2 (g)+4H.sup.+ +4e.sup.- E.sup.o =1.23 volt
results in a cell volt reduction of about one volt.
U.S. Pat. No. 4,279,711 to Vining et al teaches a similar reaction: EQU CH.sub.3 OH.sub.(aq) +H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.-E.sup.o =0.043 volt.
While these reactions are promising because of the low oxidation potential involved in all three reactions, practical applicability of these processes depend on achieving higher current densities, a more efficient consumption of carbon values in coal (for coal slurry reactions) and a more complete understanding of the effect of ash content in coal on the electrowinning kinetics and deposit morphology. No cell design in the prior art permits the full-scale commercial utilization of such a process.
Other studies have taught the simultaneous oxidation and reduction of metal ions in solution (e.g., the oxidation of Mn.sup.2+ and MnO.sub.2 and Zn.sup.2+ and Zn). In these studies, however, individual half-cells were not divided by a membrane or other separation device, and temperatures of 40.degree.-50.degree. C. above that necessary for efficient metal deposition were required which represented an additional cost factor offsetting savings.
While the prior art methods have taught ways of substituting anodic reactions other than water oxidation in the electrowinning of metals to reduce energy consumption, they have not successfully and economically taught a method where the anodic reaction is an electrochemical non-depolarizing reaction using an electrolyte where the evolved hydrogen is used to carry current through a permselective membrane wherein the membrane both permits the hydrogen ions to pass but also separates the anolyte and catholyte, and where the membrane further permits separate electrochemical and electrowinning reactions to occur at current densities compatible with an economical electrolytic cell. No prior art method has taught an anodic electrochemical reaction involving the oxidation of sodium chlorate to sodium perchlorate, although it is to be understood that the invention is not limited to this reaction and depends only on the compatibility of an electrowinning and electrochemical reaction, given constraints of cell geometry and current densities. Finally, no prior art has taught the use of two half-cell reactions which are combined within a functional cell geometry for efficiency and whose voltage together is lower than the combined individual voltages of each of these commercial half-cell reactions.